1. Technical Field of the Invention
The present invention relates to the transmission of signaling data over an air interface between a mobile station transceiver and a base station transceiver, and more particularly, to a method for protecting both network independent clocking information and status data transmitted over an air interface.
2. Description of Related Art
The GSM standard for synchronous transparent data service transmission defines two functions enabling communications between a cellular and external network. These functions enable a GSM system to accommodate a small amount of internal transmission modes and various interworking needs. The interworking function (IWF) located at the boundary between a GSM system and an external network such as a PSTN acts as an interface between the PLMN and the PSTN. On the mobile station side, the terminal adaptation function (TAF) performs the adaptation between specific terminal equipment (TE) and generic radio transmission functions.
The IWF is connected to a modem and routes a data stream to and from a PLAN using CCITT V.110 frames. The CCITT V.110 frames include data, control and status information for controlling the interconnection between the PLMN and PSTN networks. The V.110 frames are transferred through the base transceiver system (BTS) where they are channel coded and interleaved before transfer to the mobile stations (MS) over the air interface. The status and control information in the V.110 frames are handled as traffic data by the BTS and transmitted over a traffic channel. Thus, the BTS is transparent for the contents of the V.110 frames. The status and control information includes both status information and network independent clocking (NIC) information.
The network independent clocking information within the V.110 frame provides a way to control wander between the PLMN and the user generated modem signal. The wander is created due to the fact that the user generated modem signal does not have to be synchronized with the PLMN. The frequency tolerance between the modem signal and the PLMN is defined via a maximum of 100 ppm which for a 9.6 Kbit service would be approximately one bit per second. The NIC defines means for inserting or deleting a defined bit into the data stream to enable adjustment for differences between the clock rate of a user modem and the PLMN.
Status information is used for flow problem control and modem status. While this information is not as critical as the NIC information, it is important to maintain a proper flow of status data to maintain the modem connection.
A problem occurs when NIC or status information is transferred over the air interface. The air interface is subject to bit error rates at a significantly higher level then is present over the PLMN. Even with the use of redundancy coding of the information, there is still a high risk for misinterpretation of the NIC or status information when decoded. There is also the problem over a GSM air interface channel with the fast associated control channels (FACCH) which have the ability to steal bit traffic from the traffic channels over the air interface in order to assist in quickly needed call transfer procedures such as cell handovers.
These affects make it highly possible to misinterpret or completely miss either the NIC or status information. This is especially true during a FACCH steal from the traffic channel. This critical situation occurs when a call handover is performed, and an unusually high bit error rate in addition to bit stealing is present due to the performance of FACCH signaling. The affect of a missed NIC adjustment or misinterpreted NIC adjustment during handover or any other period can be fatal. The data stream would be offset between the IWF and the TAF causing the entire data streams to be misinterpreted by the receiver. Thus, there exist a need for preserving the integrity of the NIC and status information being transmitted over the air interface.